Air-conditioning systems for vehicles are generally known. They serve primarily to cool the vehicle's passenger space in the summer and are unsuitable for operation in the winter. However, in modern automobiles with engines having low fuel consumption, the heat produced by the engine is insufficient for passenger comfort in the winter. In such vehicles, in particular, to remedy this problem auxiliary heaters are supplied as standard. In principle, it is also possible to use air conditioners for auxiliary heating, i.e., to operate them in a so-called heat-pump arrangement. This, of course, amounts to a reversal of the function of conventional air conditioners employing R 134a as the refrigerant. In order to operate conventional air conditioners as heat pumps, a relatively elaborate rearrangement of the circulation and control means is required. Furthermore, the heat output, in particular at very low outdoor temperatures, is unsatisfactory, because the vaporization pressure depends on the temperature and can fall below the atmospheric pressure. Hence, the suction density and converted output are corresponding low.